1. Field of the Invention
The present invention relates to methods and apparatus for plugging a wellbore. More particularly, the invention relates to methods and apparatus to squeeze cement through perforated casing to plug a wellbore. More particularly still, the invention relates to the perforation of casing and the squeezing of cement in a single trip. The invention further relates to a firing head capable of being actuated by different means.
2. Background of the Related Art
In the oil and gas industry, plugging operations are often performed to seal wellbores in order to abandon the wells. These xe2x80x9cplug and abandonmentxe2x80x9d techniques are required under various state and federal laws and regulations. Plug and abandonment operations performed upon a cased wellbore require that at least a section of the wellbore be filled with cement to prevent the upward movement of fluids towards the surface of the well. To seal the wellbore, a bridge plug is typically placed at a predetermined depth in the wellbore and thereafter, cement is injected into the wellbore to form a column of cement high enough to ensure the wellbore is permanently plugged.
In addition to simply sealing the interior of a wellbore, plug and abandonment regulations additionally require that an area outside of the wellbore be sufficiently blocked to prevent any fluids from migrating towards the surface of the well along the outside of the casing. Migration of fluid outside the casing is more likely to arise after a fluid path inside the wellbore has been blocked. Additionally, where multiple strings of casing are line a wellbore, the annular area between the concentric strings can form a fluid path in spite of being cemented into place when the well was completed. Bad cement jobs and weakening conditions of cement over time can lead to paths being opened in the cement adequate for the passage of fluid.
In order to ensure the area outside of the wellbore is adequately blocked, cement is typically xe2x80x9csqueezedxe2x80x9d through perforations into the formation surrounding the wellbore. By pumping cement in a non-circulating system, a predetermined amount of cement may be forced into the earth and can thereafter cure to form a fluid barrier.
The perforations utilized in a cement squeeze operation are typically formed for squeezing cement. Perforations are formed with a perforating assembly that includes a number of shaped charges designed to penetrate the casing wall and extend into a formation therearound. Recently, advances in perforating have led to the development of perforating apparatus including biased members that remain in contact with the casing wall as the apparatus is lowered into the wellbore and ensure that the shaped charges remain at a predetermined distance from the wall of the wellbore. Perforating guns that are expanded and biased against the casing wall are more advantageous for making exact perforations. An example of an expanded perforating gun is described in U.S. Pat. No. 5,295,544 to Umphries, assigned to the same entity as the present invention and incorporated by reference herein in its entirety. The perforating gun includes wear plates that slide along the inner diameter of the casing and are biased against the inner wall of the well pipe casing. A string of charges are spaced about the periphery of the perforating gun. The force of the perforation is controlled by varying the standoff distance of the explosive charge from the casing wall. By controlling the spacing, it is possible to penetrate only an inner string of casing without penetrating an outer string. Furthermore, the charges can uniformly perforate all around the casing.
In a conventional plug and abandonment operation, a bridge plug or cement plug is first run into the wellbore and set therein, typically by mechanical means whereby some sealing element extends radially outward to seal the annular area formed between the outside of the device and the casing wall. Thereafter, a perforating gun is lowered into the wellbore to a pre-determined depth and discharged to perforate the casing. The perforating gun is typically discharged by a firing head. The firing head used may be pressure actuated firing heads or mechanically actuated firing heads. After the perforations are made, the perforating gun may be retrieved. Thereafter, a cement retainer is lowered into the wellbore and set above the bridge plug. The cement retainer, like the bridge plug, acts as a packer to seal an annulus between the body of the cement retainer and the casing and isolate the area where the casing will be perforated. Cement is then supplied into the cement retainer through a run-in string of tubulars attached thereto. Utilizing pressure, cement fills the isolated area of the wellbore and also extends through the perforations into the surrounding areas in the formation. After the cement is squeezed, the run-in string is disengaged from the cement retainer. Cement is then typically deposited on the cement retainer as a final plug.
In some instances, the wellbore to be plugged and abandoned has an outer string of casing and an inner string of casing coaxially disposed therein. In these instances, an annular space between the concentric strings must be squeezed with cement to prevent the subsequent migration of fluid towards the surface of the well. The plugging operation is similar to above except that only the inner string is perforated and the cement is squeezed into the annular space between the strings.
Plug and abandon operations are also performed on a central wellbore prior to the formation of a lateral wellbore. In these cases, the lateral wellbore may be drilled from a platform that includes a cement plug remaining in the central wellbore after it has been plugged. Lateral wellbores are typically formed by placing a whipstock or some other diverter in a central wellbore adjacent a location where the lateral wellbore is to be formed. The whipstock is anchored in place and thereafter, a rotating mill disposed on drill string is urged into the casing wall to form a window therein. After the window is formed, a conventional drill bit extends out into the formation to form a borehole, which can subsequently be lined with a tubular.
There are problems with the plug and abandonment techniques described above. The biggest problem relates to the number of trips into the wellbore required to adequately complete a plug and abandonment job. Another problem relates to the poor quality of perforations that are made in casing using conventional perforating apparatus. Another problem still, relates to failed firing heads on perforating guns.
Since the conventional perforating assembly has only one firing head attached, failure of the firing head to actuate can mean significant increases in costs and delays. For example, when the firing head does not actuate and ignite the perforating charges, the perforating assembly must be retrieved and the firing head replaced. Consequently, an extra run into the wellbore is necessitated by the failure. One solution is to attach two firing heads, each requiring a different type of actuation, to the perforating assembly so one may act as a backup. For instance, when a drop bar fails to acquire sufficient energy to actuate a mechanically actuated firing head, the wellbore may be pressurized to actuate the backup pressure actuated firing head and discharge the perforating assembly without retrieving the firing assembly. However, an additional firing head means additional space, weight and cost. Also, when the perforating assembly is discharged by the intended firing head, the backup firing head is necessarily destroyed in the explosion.
There is a need therefore to uniformly perforate the casing to squeeze cement into the intended areas in an efficient and effective time saving manner.
The present invention provides a method and apparatus for plugging a wellbore in a trip saving manner. In one aspect, the invention includes a cement retainer disposed on a run-in string and a radially expanded perforating assembly disposed below the cement retainer. In a single run, the apparatus provides for perforating a wellbore and squeezing cement through the perforations and into the formation therearound. In another aspect, a method of plugging the wellbore includes running a cement retainer and a radially expanded perforating assembly into a wellbore on a run-in string. After the cement retainer is set, a firing head is actuated to cause the perforating gun to discharge. After perforations are formed, cement is introduced from the cement retainer into the isolated area and squeezed through the perforations. Thereafter, the run-in string disengages from the cement retainer leaving behind the plug formed. In yet another aspect, a firing head capable of being actuated by different means is used to discharge the perforating assembly.